Genesis of BRO β‐lactamase‐producing Moraxella catarrhalis: evidence for transformation‐mediated horizontal transfer

Abstract

SummaryThe dramatic rise in BRO-producing M. catarrhalis strains observed in the last decades is without precedence. The aim of this study was to elucidate the events that led to the emergence of BRO-1 and BRO-2 b-lactamases. Previously, we showed bro1 and bro2 to be .99% identical. Data presented here suggested that bro2 was acquired by a fortuitous event and inserted between M. catarrhalis genes orf1 and orf3. Subsequently, bro1 evolved from bro2. Promoter-up mutations increased fitness of bro2, explaining its present predominance. The highly conserved nature of bro compared with orf1 and orf3 suggested that acquisition has occurred relatively recently. The random distribution of bro among M. catarrhalis fingerprint types indicated that bro has spread by horizontal transfer. Sequence analysis revealed that 80±200 bp is generally cotransferred with bro, serving as regions of homology that target bro to the same chromosomal locus. A region of 160 bases upstream of bro1 lacked polymorphism, indicating it was derived from the original strain that acquired bro2. We observed that bro was readily transferred by transformation between M. catarrhalis strains in vitro, suggesting a mechanism by which bro has disseminated. In conclusion, we have been able to reconstruct the steps that led to the emergence of BRO-producing M. catarrhalis.

Natural insertion of the bro-1 β-lactamase gene into the gatCAB operon affects Moraxella catarrhalis aspartyl-tRNAAsn amidotransferase activity

“…1). This supports the hypothesis of Bootsma et al (2000) that bro-2 arrived first and evolved into bro-1 by a point mutation. Strains carrying bro-1 then obtained a selective advantage by other mutations involving the bro promoter, and were disseminated by lateral transfer by transformation rather than by clonal expansion (Bootsma et al, 2000).…”

“…It is known that M. catarrhalis does not contain mobile elements associated with bro and has disseminated the bro gene by natural transformation (Bootsma et al, 2000). Analysis of the complete genomic sequences of 12 M. catarrhalis isolates (Davie et al, 2011) using gapped BLAST and the Genetics Computer Group v11 software (Accelrys) revealed the presence of genes corresponding to GatCAB, GluRS, GlnRS and AspRS, but no gene for AsnRS was found.…”

“…The G+C content is about 31 and 42 % for the bro gene and the M. catarrhalis genome, respectively, suggesting that the bro gene was recently laterally transferred from another organism (Bootsma et al, 2000). The two GatCs are well conserved, as well as the two GatBs, but there is a significant difference between the GatA of the sensitive strain (GatA wt ) and the GatA of the resistant strain (GatA BRO ).…”

“…Two closely related types of bro genes have been described based on a difference in their isoelectric focusing patterns attributable to a single amino acid residue difference at position 293, Asp in BRO-1 and Gly in BRO-2 (Bootsma et al, 1996). Eight of the 12 M. catarrhalis genome sequences had bro-1 sequences and three genomes had bro-2 sequences; one genome was wild-type (sensitive).…”

“…M. catarrhalis underwent a rapid evolution from ,10 to .90 % ampicillin resistance in the space of a few years (Doern et al, 1996;Wallace et al, 1989). Most of this resistance is due to the insertion of the bro-1 b-lactamase gene into the chromosome (Wallace et al, 1990); less frequently, the resistance is due to the BRO-2 b-lactamase, which differs by one amino acid (Bootsma et al, 1996). Surprisingly, the upstream and downstream neighbours of the bro-1 gene were identified in the characterized bro-1 M. catarrhalis strains as the gatA and gatB genes encoding AdT subunits (D. Beaulieu and P. H. Roy; GenBank file U49269).…”